AD7249
Internal Reference
Op Amp Section
The AD7249 has an on-chip temperature compensated buried
Zener reference which is factory trimmed to 5 V
±
50 mV. The
reference voltage is provided at the REFOUT pin. This refer-
ence can be used to provide the reference voltage for the D/A
converter by connecting the REFOUT pin to the REFIN pin.
The reference voltage can also be used as a reference for other
components and is capable of providing up to 500
µA
to an
external load. The maximum recommended capacitance on
REFOUT for normal operation is 50 pF. If the reference output
is required to drive a capacitive load greater than 50 pF, then a
200
Ω
resistor should be placed in series with the capacitive
load. Figure 3 shows the suggested REF OUT decoupling
scheme, a 200
Ω
resistor and the parallel combination of a
10
µF
tantalum and a 0.1
µF
ceramic capacitor. This decoupling
scheme reduces the noise spectral density of the reference.
The output of the voltage-mode D/A converter is buffered by a
noninverting CMOS amplifier. The R
OFS
input allows three
output voltage ranges to be selected. The buffer amplifier is
capable of developing +10 V across a 2 kΩ load to AGND.
The output amplifier can be operated from a single +15 V sup-
ply by tying V
SS
= 0 V.
The amplifier can also be operated from dual supplies to allow
an additional bipolar output range of –5 V to +5 V. Dual sup-
plies are necessary for the bipolar output range but can also be
used for the unipolar ranges to give faster settling time to volt-
ages near 0 V, to allow full sink capability of 2.5 mA over the
entire output range and to eliminate the effects of negative offsets
on the transfer characteristic (outlined previously). A plot of the
output sink capability of the amplifier is shown in Figure 5.
3
REFOUT
200
EXT LOAD
10 F
0.1 F
2
V
SS
= –15V
I
SINK
– mA
V
SS
= 0V
Figure 3. Reference Decoupling Scheme
External Reference
1
In some applications, the user may require a system reference or
some other external reference to drive the AD7249. References
such as the AD586 provide an ideal external reference source
(See Figure 10). The REFIN voltage is internally buffered by a
unity gain amplifier before being applied to the D/A converter.
The D/A converter is scaled for a 5 V reference and the device is
tested with 5 V applied to REFIN. Other reference voltages may
be used with degraded performance. Figure 4 shows the degra-
dation in linearity vs. REFIN.
1.0
0.9
0.8
LINEARITY ERROR – LSBs
0
0
2
4
6
OUTPUT VOLTAGE – Volts
8
10
Figure 5. Amplifier Sink Current
500
V
DD
= +15V
V
SS
= 0V
T
A
= +25 C
200
REFERENCE
(NO DECOUPLING)
nV/ Hz
100
REFERENCE
(DECOUPLED*)
50
V
DD
= +15V
V
SS
= –15V
T
A
= +25 C
0.7
0.6
0.5
20
0.4
INL
0.3
0.2
DNL
0.1
0.0
2
3
4
5
6
REFIN – Volts
7
8
9
OUTPUT WITH ALL
0s ON DAC
0
50
100
200
500 1k
2k
5k 10k
FREQUENCY – Hz
20k
50k
100k
* REFERENCE DECOUPLING COMPONENTS ARE
A 200 RESISTOR IN SERIES WITH A PARALLEL
COMBINATION OF 10 F AND 0.1 F TO GND.
Figure 6. Noise Spectral Density vs. Frequency
Figure 4. Linearity vs. REFIN Voltage
–6–
REV. C
AD [ ANALOG DEVICES ]
